1. Field of the Invention
The invention relates to compositions that are blends of ultrahigh molecular weight polyethylene (UHMWPE) and high-density polyethylene (HDPE) which are capable of being melt spun. The invention includes the method of melt spinning and the fibers produced thereby.
2. Description of the Related Art
HDPE is generally considered to encompass linear polyethylene having weight average molecular weights in the range from about 20,000 to about 250,000 Daltons. UHMWPE is generally considered to encompass linear polyethylene having weight average molecular weight exceeding about 500,000 Daltons.
Melt spinning of HDPE to produce strong fibers is well known in the art and is taught, for example, in U.S. Pat. Nos. 4,228,118 and 5,474,845. Melt spinning is an easily and economically practiced technology. Attainable fiber strength and resistance to creep under sustained loads are known to increase with molecular weight; however so also does melt viscosity. The very high melt viscosity of UHMWPE has prevented its melt spinning in a conventional manner, either because required extrusion pressures were beyond equipment capabilities, or because melt fracture occurred producing rough filaments.
Solution spinning of UHMWPE to produce very strong fibers is also well known in the art and is taught, for example, by U.S. Pat. Nos. 4,413,110 and 6,448,359. Solution spinning bypasses the limitations of the high melt viscosity of UHMWPE, but requires circulation and recovery of large volumes of solvent with attendant high capital and operating costs.
To address the problem of high melt viscosity, U.S. Pat. No. 5,234,552 describes a process wherein an UHMWPE is extruded through a die with a lubricating material being injected through a ring just prior to the die. The lubricant disclosed was a low molecular weight ethylene-acrylic acid copolymer trade named “ACCUFLOW”.
U.S. Pat. No. 5,256,358 describes a process for the extrusion of a single strand of UHMWPE.
U.S. Pat. No. 4,281,070 describes an UHMWPE blend containing an intermediate molecular weight polyethylene in a molecular weight range of from about 400,000 to 600,000, and a finely divided nucleating agent such as pyrogenic silica. The blend was processable by injection molding and is said to be processable with conventional melt forming equipment such as screw extruders. The patent teaches away from blends of intermediate molecular weight polyethylene and UHMWPE.
U.S. Pat. No. 6,599,982 describes the use of fluoroelastomers to improve the extrusion processability of polyethylene. It indicates that mineral particulates have a deleterious effect.
An investigation titled “Thermal, Mechanical, and Rheological Behavior of Blends of Ultrahigh and Normal-Molecular Weight Polyethylenes”, S. K. Bhateja et. al., Poly. Eng. & Sci., 23(16), 888-893 (1983), concluded that the addition of normal molecular weight linear polyethylene even up to 40 weight percent does not sufficiently improve the flow behavior of UHMWPE to make it amenable to conventional melt processing.
In an article titled “Extrusion Instabilities and Wall Slip”, M. M. Denn, Annual Review of Fluid Mechanics, 33,365-287(2001), it is suggested that extrusion instabilities such as melt fracture are related to the onset of wall slip. Denn reported onset of roughness in low density linear polyethylene at a wall stress of the order of 0.1 MPa and unsteady flow at wall a stress on the order of 0.3 MPa.
Although each of these references represented a contribution to the state of the art, a need still exists for a polyethylene composition that can exploit the advantages of UHMWPE and yet is melt spinnable into multi-filament fiber and yarn.